1. Field of the Invention
The present invention relates to an electrical distribution system and a method for controlling an electrical distribution system.
2. Prior Art
It is a common practice to design a PV installation with an array-to-inverter ratio above 100%. That means that in high irradiance and/or low temperature conditions the array is able to produce more power that the inverter is able to handle. This is done because most of the time the array will operate below the maximum theoretical power, and therefore, if the ratio is selected at 100%, most of the time the plant will be producing less power than it is rated for.
Due to the rating of the inverter or because of limits in the interconnection agreement, the inverter commonly should not convert more power than a design value Pgrmax. Consequently, when the array is capable of operating at a power above Pgrmax, the inverter will limit or “clip” the power. In addition, there is the trend to establish dynamic limits to power produced by PV power plants so that a central utility controller can limit the power being injected to the grid depending on the balance between generation from conventional plants and load fluctuations. In this case, Pgrmax would be changed usually in steps by an external command to the inverter and the inverter should “clip” the generated power to this variable limit.
In conventional applications, the clipping is executed by the inverter. The way the inverter clips the power is by reducing the current drawn from the PV array so that the operation point goes away from the maximum power point and a power below the maximum is injected to the grid. In a distributed PV system with DC/DC converters transferring power between groups of PV modules and a common DC-link like the one in FIG. 1, the maximum power point tracking (MPPT) algorithm is executed by the DC/DC converters but the interconnection with the grid is executed by the inverter. The power from the generators has to be clipped by the DC/DC converters and not by the inverter. To achieve the power clipping in a distributed system using DC/DC converters in a simple way, each DC/DC converter is provided with a pre-established maximum power setting.
This method does not maximize the power production from the complete installation. This is because some DC/DC converters may unnecessarily be clipping the power. This case is explained in FIG. 2 where a maximum power of 30 kW can be produced by an installation with 3 DC/DC converters such that a 10 kW clipping is set on each of the converters. At some instant, the array connected to CNV1 may be capable of producing power 12 kW while the array connected to CNV2 is shaded and only able to produce 6 kW, and the array connected to CNV3 is able to produce 10 kW. CNV1 would then clip its power to 10 kW but then the complete installation would produce 26 kW instead of the maximum 30 kW specified. Due to the shading of CNV2, it would be acceptable to operate CNV1 at 12 kW without exceeding the total power limit for the installation but CNV1 has a power clipping limit that does not take in account the power produced by the other converters. Another limitation of this simple method is that the maximum power cannot be easily changed after the system is commissioned so that the dynamic power limit is not possible.
A known method to overcome this weakness is to use a communication network to actively clamp the power from each converter based on the power measured by the inverter. In this method, the inverter appropriately controls each converter when power clipping is required via the communication network. This strategy can be complex, slow in response and costly to implement since a reliable communication link between the inverter and the DC/DC converters is needed.